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Spare 94mm piston anyone?
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jslew
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PostPosted: Tue Sep 22, 2015 10:27 pm    Post subject: Spare 94mm piston anyone? Reply with quote

I recently got a 79 FI bus which had been sitting for 7 years. Took the engine apart to check clearances & see what kind of shape its in. Crank play & rod/crank clearance are excellent, and 3 out of 4 piston/cylinder clearances are excellent (.0017-.0019). One piston is .001 larger in diameter than the others, and the clearance in that cylinder is .0006!! No evidence it ever seized... maybe it did and that's why it was parked??
I was wondering if anyone out there for whatever reason has ended up with a spare 94mm piston in good shape? I need one that measures (as per bentley) anywhere from 3.6965" - 3.6972". I would also be interested in a single 94mm piston/cylinder combo if anyone has one laying around, but would want to know the clearance.
Thanks! -Jeff
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PostPosted: Wed Sep 23, 2015 12:34 am    Post subject: Reply with quote

I have a full set in good condition.
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jslew
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PostPosted: Wed Sep 23, 2015 9:08 am    Post subject: Reply with quote

New ones are easy. But I'm a cheap VW owner Wink , and would rather not pay $400 a set just to get one piston! Especially not when the other 3 are excellent.
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SGKent Premium Member
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PostPosted: Wed Sep 23, 2015 9:31 am    Post subject: Reply with quote

take it to a good machine shop. Also when you measure it. Do so just below the wrist pin 90 degrees to the wrist pin. Do not measure them on the skirts as skirts collapse, or bend outwards sometimes if dropped.

Another option is measure all 4 cylinders and match them by clearance because it is possible someone swapped cylinders when it was built. Once you are done matching them by size, you will want to have them honed on a sunnen cylinder honing machine to even them up. If you are still tight on one or two, the shop can hone a little more out to give you the correct clearances. If you try to find a matching piston you will have other issues where the wrist pin to deck height can vary, weight differences can be excessive etc. Normally pistons don't wear out for a long time, it is the ring lands that wear away, and skirts collapse inwards. Before you decide those are good, make sure you measure the ring land clearance, and how much the skirts have collapsed.
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my59
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PostPosted: Wed Sep 23, 2015 3:36 pm    Post subject: Reply with quote

I have a set out of 79 that has one piston with a broken section between the rings. It was the second piston i was de-ringing and after that i said screw this and bought a new set. They came out of an engine with low/no oil pressure.
Send me a box postage paid for the weight of 4 P&C big enough for pistons out of the cylinders and I will put em all in and send the box back.
It will save me from dragging the milk crate to the dump and hurling everything into the scrap metal bin.
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jslew
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PostPosted: Wed Sep 23, 2015 3:51 pm    Post subject: Reply with quote

my59 wrote:
I have a set out of 79 that has one piston with a broken section between the rings...


PM sent
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jslew
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PostPosted: Wed Sep 23, 2015 6:55 pm    Post subject: Reply with quote

SGKent wrote:
take it to a good machine shop. Also when you measure it. Do so just below the wrist pin 90 degrees to the wrist pin. Do not measure them on the skirts as skirts collapse, or bend outwards sometimes if dropped.


My neighbor runs a machine/speed shop. He questioned if taking .001" out of a cylinder might be too much? The cylinders are all the exact same diameter, so moving pistons wont really help.
I actually measured the piston in several places. No more than .0003" difference in roundness. I swear someone stuck in a bigger piston!
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Tcash
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PostPosted: Wed Sep 23, 2015 7:20 pm    Post subject: Reply with quote

jslew wrote:

He questioned if taking .001" out of a cylinder might be too much?
You can hone .001 out of the cylinder no problem. Oversize pistons came in .020".
I would be more concerned about the weight. Balance if necessary.


Good luck
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PostPosted: Wed Sep 23, 2015 7:31 pm    Post subject: Reply with quote

Tcash wrote:

I would be more concerned about the weight. Balance if necessary.


Pistons came in several weight groups. It's hard enough to effectively take one piston down an entire weight group, let along three…

Good luck, seriously!
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jslew
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PostPosted: Wed Sep 23, 2015 8:06 pm    Post subject: Reply with quote

I still need to weigh what i've got. I'll do that & go from there. Sounds like easiest fix (if weight is close) is to just take .001 out of the cylinder. We can adjust piston weight.
So, at some point maybe just that piston got replaced? I wonder how many miles it did w/ .0006" clearance!!
Thanks guys.
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Tcash
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PostPosted: Thu Sep 24, 2015 9:42 am    Post subject: Reply with quote

jslew wrote:

So, at some point maybe just that piston got replaced?


More likely that it came with the set. Someone just missed it or let it go.
You have to imagine a worker with a pile of pistons in front of him and miking them and sorting them by size and weight. .001 would be easy enough to miss or they just did not give a ?hit.

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SGKent Premium Member
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PostPosted: Thu Sep 24, 2015 1:51 pm    Post subject: Reply with quote

jslew wrote:
I still need to weigh what i've got. I'll do that & go from there. Sounds like easiest fix (if weight is close) is to just take .001 out of the cylinder. We can adjust piston weight.
So, at some point maybe just that piston got replaced? I wonder how many miles it did w/ .0006" clearance!!
Thanks guys.


the problem I have with this when I think about it is that .0006 is almost an interference fit. I'd make all my measurements again at room temp and make sure I was holding the mic properly so my hand did not warm it. Then I would use a good quality bore gauge to measure the cylinders.

Regardless, the machine shop can sort it all out when then hone the cylinders. Use a Sunnen Cylinder hone and not one on a drill or a bottle brush. The stones in the Sunnen will keep the cylinders straight. You'll just have to rig a way to hold them down and use light pressure when honing.
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